Drop-on-demand liquid emission devices with electrostatic actuators are known for ink printing systems. U.S. Pat. No. 5,644,341 and U.S. Pat. No. 5,668,579, which issued to Fuji et al. on Jul. 1, 1997 and Sep. 16, 1997, respectively, disclose such devices having electrostatic actuators composed of a single diaphragm and opposed electrode. The diaphragm is distorted by application of a voltage differential between two electrodes. Relaxation of the diaphragm expels an ink droplet from the device. Other devices that operate on the principle of electrostatic attraction are disclosed in U.S. Pat. No. 5,739,831, U.S. Pat. No. 6,127,198, and U.S. Pat. No. 6,318,841; and in U.S. Publication No. 2001/0023523.
U.S. Pat. No. 6,345,884 teaches a device having an electrostatically deformable membrane with an ink refill hole in the membrane. An electric field applied across the ink deflects the membrane and expels an ink drop.
IEEE Conference Proceeding “MEMS 1998,” held Jan. 25-29, 2002 in Heidelberg, Germany, entitled “A Low Power, Small, Electrostatically-Driven Commercial Inkjet Head” by S. Darmisuki, et al., discloses a head made by anodically bonding three substrates, two of glass and one of silicon, to form an ink ejector. Drops from an ink cavity are expelled through an orifice in the top glass plate when a membrane formed in the silicon substrate is first pulled down to contact a conductor on the lower glass plate and subsequently released. There is no electric field in the ink. The device occupies a large area and is expensive to manufacture.
U.S. Pat. No. 6,357,865 by J. Kubby et al. teaches a surface micro-machined drop ejector made with deposited polysilicon layers. Drops from an ink cavity are expelled through an orifice in an upper polysilicon layer when a lower polysilicon layer is first pulled down to contact a conductor and is subsequently released.
One such device is disclosed in co-pending U.S. patent application Ser. No. 10/155,306 filed in the names of Gilbert A. Hawkins and James M. Chwalek on May 23, 2002. That device includes an electrostatic drop ejection mechanism that employs an electric field for driving liquid from a chamber in the device. Structurally coupled, separately addressable first and second dual electrodes are positioned on opposed sides of a third electrode. The first and second electrodes are movable in a first direction to draw liquid into the chamber and in a second direction to emit a liquid drop from the chamber.
In above-mentioned U.S. Pat. No. 6,127,198, air trapped between the distortable diaphragm and the opposed, fixed electrode is compressed when a voltage is applied to the electrode. The air chamber must have a relatively large volume to accommodate the compressed air; reducing the number of ejection nozzles that can be located in a given area.
U.S. Pat. No. 6,235,212 provides a vented space between a distortable diaphragm and the opposed, fixed electrode. The vent is a very thin slot around the perimeter of the device. Because the mechanism relies on hydrophobic layers between the electrodes to keep the chamber clear of fluid, the cross-sectional area of the perimeter vent gap is by necessity insufficient to provide adequate venting. The thickness of the vent is given in the patent as 0.5 μm. Even assuming that the entire perimeter on an 80 μm device were vented (although it is likely that, say, 25% of the perimeter would be used to anchor the device), the area of the vent would be only about 120 μm2; as calculated below by approximating the area as the surface area of a cylinder:                               2          ⁢          π          ⁢                                           ⁢          r          *          thickness                =                ⁢                  2          ⁢          π          *          40          ⁢                                           ⁢                      µ                    ⁢                                           ⁢          m          *          0.5          ⁢                                           ⁢                      µ                    ⁢                                           ⁢          m                                        ≅                ⁢                  120          ⁢                                           ⁢                      µ                    ⁢                                           ⁢                      m            2                              The perimeter of the vent would be approximately 240 μm, for an area-to-perimeter ratio of 0.5 μm. This would be a very slowly venting device; and therefore would be slow to fire and refill.